A city of limestone towers rises in western Madagascar.Photo: Stephen Alvarez / National Geographic

Benson weaves through skin-ripping pinnacles. In Malagasy, the formations are called tsingy, meaning "where one cannot walk barefoot." The terrain resists intrusions from hunters, hungry cattle, and wildfires.Photo: Stephen Alvarez / National Geographic

A couple of weeks ago, we featured a story of how NatGeo photographer Stephen Alvarez's went deep underground to explore the caves in the corner of Tennessee, Alabama, and Georgia.

This time, Stephen, along with Neil Shea and biologist Hery Rakotondravony and colleagues went the opposite way - they climbed Madagascar's astonishing Tsingy de Bemaraha stone forest:

One afternoon, returning from a hot, wet slog, vines along the trail tripped me up, and my right knee landed on a small rock. Back home in New England, where rocks come in rounder forms, I would have walked away with a bruise. But this was tsingy in miniature. A barb of limestone drove in nearly to the bone. It took two days to reach a hospital, where a nurse removed dirt from the wound. "Why were you doing this?" she asked, twisting a swab deep into the hole. She looked up. I was sweating. "I think you are a little dumb," she said. The tsingy is the perfect foil to human ambition.